The sea assault is also slower than it looks – if you build a factory on G1, and a navy on G2, you still can’t get your troops to Ukraine / Caucasus any faster than G3, which means you can’t take the factory in Volgograd before G4.
Even if you’re just marching overland as normal, you can get to Eastern Poland / Bessarabia on G1, Ukraine on G2, Rostov on G3, and Volgograd on G4. The navy gives you a bit of tactical flexibility, but it doesn’t give you any extra speed, and it’s wicked expensive.
If you have extra Italian boats in the Med because the British refused to attack there or something like that, I could see the Italians diverting a transport and a destroyer to the Black Sea to have some fun and do some can-opening for German tanks, but a German Black Sea navy is a huge waste of time – and I say this as someone who thinks a factory in Romania can make sense as a way to get some closer infantry!